Heretofore, as liquid crystal compounds which can suitably be used in a super-twisted nematic device (heretofore abbreviated to "the STN device" sometimes), various compounds having an alkenyl group as a side chain have been synthesized, and some of these compounds have practically been used. For example, compounds represented by the following formulae (s-i) to (s-3) are disclosed in Mol. Cryst. Liq. Cryst., Vol. 122, p. 241 (1985) and Japanese Patent Application Laid-open No. 83136/1986, compounds represented by the following formulae (s-4) and (s-5) are disclosed in Japanese Patent Application Laid-open No. 92740/1994, and compounds represented by the following formulae (s-6) and (s-7) are disclosed in DE 19520246A1. Furthermore, compounds represented by the following formulae (s-8) and (s-9) in which a fluorine atom is directly bonded to a double bond are disclosed in Japanese PCT Patent Application Laid-open No. 500343/1994, and a compound represented by the following formula (s-10) is disclosed in Japanese PCT Patent Application Laid-open No. 502627/1992. ##STR2##
However, the compounds represented by the formulae (s-1) to (s-7) all have strong inclination to show a smectic phase, so that it is difficult on occasion to use a large amount of the compound in a liquid crystal composition. In addition, the compounds represented by the formulae (s-4) to (s-7) have a high viscosity, and the compounds represented by the formulae (s-8) to (s-10) contain some portions which should be improved. For example, they are poor in chemical stability (particularly heat stability) on occasion. In particular, the improvement of a steepness of electro-optic characteristics is still desired.